Smectic liquid crystals (LC) are materials that, upon cooling from the isotropic (liquid) phase and before solidification, can form a liquid crystalline phase having a layered structure in which the liquid crystal molecules show various morphologies within the layers. The various morphologies give rise to a number of different smectic phases designated by the letters A, B, C, etc. the most common of which is smectic A (SmA). The SmA phase has LC molecules orthogonal to the layers but randomly distributed within the layers it is therefore one of the least ordered of the Smectic phases. X-ray studies show a weak density wave, characteristic of well defined layer spacing and the materials are distinct from nematic phases both via microscopy and in visco-elastic and other properties.
Most optically inactive SmA materials are linear and rod-like (sometimes referred to as “lath”-like) and usually have a positive dielectric anisotropy, i.e. the average direction of the long molecular axis (indicated by a vector called the ‘director’) of the molecules will align with the gradient in an electric field applied across a film of smectic A liquid crystals present between two substrates (e.g. made of glass). If the substrates act as electrodes sandwiching the film, the directors will then lie orthogonal to the substrates. This orientation is referred to as ‘homeotropic alignment’.
“8CB” (4′-octyl-4-biphenylcarbonitrile) and “8OCB” (4′-(octyloxy)-4-biphenylcarbonitrile) are examples of materials that exhibit a SmA phase when cooled from the higher temperature nematic phase.
In the homeotropic alignment, all the SmA LC molecules are arranged in layers between the substrates; the layers extend in a direction parallel to the substrates and, as stated above, the directors of individual LC molecules are substantially orthogonal to the plane of the substrates. The homeotropic SmA structure can be disrupted and broken up into domains and the uniform structure will be maintained separately in each domain. The greater the number of domains, the less ordered the state of the SmA material will be and conversely the smaller the number of domains, the more ordered the material will be. Even in an extreme case of disorder, the orientation of the layers in the different domains will not be completely random.
SmA liquid crystals are optically anisotropic and so the greater the number of domains present, the greater the light scattering. This is sometimes referred to as the ‘scattering state’. In a uniform homeotropic state, a SmA composition will appear transparent and this is sometimes referred to as the ‘clear state’. However, the same SmA composition, when in the scattering state, in suitably thick samples can scatter light to such an extent that it is opaque.
A test cell containing the SmA composition may be formed by taking planar glass sheets coated with a transparent conducting layer, typically made of indium tin oxide. These two sheets may be formed into a thin cell for example separated by spacers of uniform diameter (typically, above 5 micrometers, dependent on the desired cell thickness). This cell is normally edge sealed with glue allowing apertures for filling. A SmA liquid crystal layer may be formed by filling the cell (typically at an elevated temperature above the isotropic transition for the material). The application of wires to contact the conducting glass allows a field to be applied across the liquid crystal layer. In the SmA devices discussed here, no alignment layers need be used, in strong contrast to nematic display-type devices where uniform alignment of the cell is a requisite of their operation.
To electrically address a SmA liquid crystal cell, an alternating (AC) field is normally applied in order to avoid damage to the liquid crystal by electrochemical reactions at the electrodes as are normally obtained using DC (and low frequency AC <50 Hz). For materials that do not contain ionic dopants (explained below), the dielectric anisotropy of the LC will cause them to rearrange and align with the applied field direction (normal to the substrate surface). Under such a condition, the cell (viewed in transmission or normal to its surface), will typically appear clear. The SmA material is now in an ordered mono-domain, with the layers of the material lying parallel to the substrate and the directors of the individual LC molecules lying orthogonal to the layers and to the substrates. For many SmA materials this situation is only reversible by re-heating the cell to a nematic phase and so destroying the SmA alignment.
Because the switching from a clear state to a scattering state can only be reversed by such heating and subsequent cooling, SmA liquid crystals, with positive dielectric anisotropy, cannot alone form the basis of a practical electro-optic phenomenon. However a light scattering state can be electrically induced from a mono-domain clear state by smectic dynamic scattering (SDS), as described below, that disrupts the mono-domain state to form multi-domains, which allows a display to be reversibly switched between a homeotropically aligned clear transparent state and a disordered light scattering state. These two states are visible without polarised light.
Smectic dynamic scattering uses a suitable ionic dopant that is dissolved in the smectic A liquid crystal host; under the influence of low frequency (e.g. <500 Hz) electric fields, two orthogonal forces attempt to reorient the SmA director. Dielectric re-orientation, as described above, attempts to align the SmA director (indicating the average direction of the long molecular axis) in the field direction, i.e. orthogonal to the plane of the electrodes/substrates. Simultaneously, the movement of dopant ions through the SmA electrolyte attempts to align the SmA director in the direction in which ions find it easier to travel. In SmA materials this direction is within the SmA layers, which lie orthogonal to the field direction, i.e. SmA materials have a “negative conductivity anisotropy”. The cumulative effects of the movement of the ionic charges leads to a field arising in the plane of the layers that attempts to align the SmA director in a direction parallel to the plane of the electrodes/substrates. The two competing forces give rise to an electro-hydrodynamic instability in the liquid crystal fluid that is referred to as ‘dynamic scattering’ or smectic dynamic scattering (SDS). If the dopant ions are present in sufficient quantity, the scattering caused by the transit of the ions dominates the dielectric orientation of the LCs, thereby forming a disordered scattering state in which the SmA material scatters light, as described above.
The reversibility between the clear, uniformly oriented state and the ion-transit induced, poly-domain, scattering state, depends upon the frequency of the applied electric field. Dynamic scattering requires the field driven passage of ions through the liquid crystal. It therefore occurs only with DC or low frequency AC drive. At higher frequencies the ions cannot react fast enough to the changing field frequency and so do not move sufficiently to induce a scattering state. However, at such higher frequencies, the dielectric reorientation of the LCs due to the electric field across the material still occurs. Therefore, if a high frequency ac field is applied to a SmA material in a disordered polydomain state, the field re-orientates the LCs thereby re-establishing a uniform orientation of the molecules in an ordered homeotropic SmA state.
One particular characteristic of SmA liquid crystals is a marked bi- or multi-stability in their switching to the extent that dielectric re-orientation (or other disturbances of the smectic structure) does not relax when the electric field is removed (see Crossland et al [P4 and ref. 6]), i.e. unlike most nematic liquid crystal structures, dielectrically re-oriented SmA liquid crystals remain in the driven state until further forces are applied.
It can therefore be seen that the combination of dielectric re-orientation (into a clear transparent state) and dynamic scattering (into a light scattering state) in a suitably doped SmA phase (possessing a positive dielectric anisotropy and a negative conductivity anisotropy) can form the basis of an electrically addressed display (and other optical devices) and is used in the present invention. High frequencies (variable, but typically ≧500 Hz) drive the SmA layer into an optically clear ordered state and low frequencies (variable, typically dc or <500 Hz) drive it into the light scattering disordered state. A key feature of such a display is that both these optical states can be set up using short electrical pulses, and both persist indefinitely, or until they are re-addressed electrically. This is not true of the related phenomena in nematic liquid crystals. It is this property of electro-optic bistability (or more accurately multi-stability since a range of different stable states are possible) that allows SmA dynamic scattering displays to be matrix addressed without pixel circuitry and which results in their extremely low power consumption in page-oriented displays or in smart windows.
The phenomenon of dynamic scattering in SmA liquid crystals was predicted by Geurst and Goosens in 1972 (ref 8). It was first observed and identified by Crossland et al 1976 (ref P1) who proposed displays based on this phenomenon and dielectric re-orientation and described their structure and electrical addressing (refs. 1-3 and P1, P2, P3). Subsequently highly multiplexed passive matrix displays were demonstrated with good viewing characteristics based on efficient switching between clear and scattering states (refs 4). The background on SmA liquid crystals as a phase of matter is widely covered in the liquid crystal literature (e.g. in the books ref 9).
SmA displays are generally viewed against a black background and could be illuminated (e.g. using a transparent plastic light guide lit at the edges) or used without artificial illumination as reflective displays. They were also used as efficient projection displays because the clear areas are highly transparent (no polarising films are needed) and the scattering textures efficiently scatter light out of the aperture of projection lenses.
A second method of generating contrast in optical devices using SmA materials and the above-described electro-optic effects was also disclosed in P1 (Crossland et al 1976): if a suitable dichroic dye is dissolved in the SmA then the dye orientation is randomised in the scattering state, which therefore appears coloured. The clear state however orients the dye absorption axis so that it lies orthogonal to the liquid crystal layer (and the direction of view) so its absorption band is not apparent and it appears colourless or only weakly coloured. This ‘guest-host’ effect (where the dye is the guest in the SmA host) switches between the dye colour and white when viewed against a white background. Displays have been fabricated using dyes of various colours (including mixtures of dichroic dyes to give black) and devices employing, for example, anthraquinone based dyes exhibited good contrast and photochemical stability.
In outdoor applications, however, sunlight tends to bleach the dyes and this shortens the lifetime of SmA displays of the type discussed above. It is this problem that the present invention addresses.
This invention relates to displays as described above, in which a disordered state is produced by the process of SmA dynamic scattering and a uniform ordered state, which (depending on the LC composition) is typically clear, is induced by dielectric re-orientation. Here they are referred to as SmA dynamic scattering (SDS) displays or devices. These two states are equally stable allowing arrays of pixels of any size to be addressed as line-at-a-time without the use of pixel circuitry. Such line-at-a-time display drivers are well known.
SmA dynamic scattering displays have not heretofore been attractive for main-stream video display development due to lifetime limitations and nematic liquid crystals have largely been favoured. However, with emergent requirements for electronic display systems of superior energy efficiency the SmA materials offer several significant intrinsic advantages. In particular SmA materials are very attractive for information displays where video performance is not requisite and high energy efficiency, and quite possibly ambient lit operation, is desired (reflective display systems).
A typical example of such a display is provided by metropolitan information systems (e.g. displays of road-traffic information, public transport timetables, visitor information etceteras). Such will need to operate in a quasi continuous up-date mode, with some sites requiring full exposure to sunlight, others being sited where frequent maintenance is difficult. Such applications will thus require refresh rates that are reasonable and provide a readable experience (for comparison, consider the experience of reading and turning a page of a book or magazine). Similarly with continuously refreshed and paged data, the expectation for acceptable lifetime must suggest that the screen can be refreshed many times, say, for a service life of 3 to 5 years (if we assume pages will be refreshed every 10 seconds then this would imply that the display must operate between 10 and 16 Million refresh cycles). Naturally this operation scenario is not the only consideration, but it does provide a useful guideline for the fabrication of practical devices.
The use of SmA SDS in reflective, and front and/or back-lit, display systems goes back to the 1970s and 1980s when early trials of SmA materials in a scattering display mode were evaluated for point-of-sale display, information systems, electronic books and electronic displays for overhead projectors (see Crossland et al ref 4). The choice between using dyed or un-dyed systems has historically been dependent upon application specifications but dyed systems have not been usable in outdoor applications because of the bleaching problems mentioned.
WO 2006/003171 (P8) discloses a liquid crystal display comprising a liquid crystal composition sandwiched between a pair of electrodes. Anisotropic light-absorbing particles, which may be colloidal particles or pigments, are dispersed within the composition and align themselves with the liquid crystals of the composition. The liquid crystal used in this disclosure was 4-pentyl-4′-cyanobiphenyl, a single compound nematic liquid crystal (K15, Merck), which is not a SmA liquid crystal material. A further indication that the device disclosed in this document is not a SmA display comes from the disclosure that the device requires an alignment layer in order to align the liquid crystals, which is not needed for aligning a SmA liquid crystal composition but is essential for aligning a nematic liquid crystal composition. Furthermore, the schematic diagrams shown do not include the characteristic smectic A “layered” structure.
WO/2011/115976 discloses a thermotropic liquid crystal smectic A composition exhibiting a smectic type A phase made up of multiple layers and capable of forming a liquid crystal optical device, e.g. a display, when sandwiched between a pair of electrodes, wherein:
under the influence of different electric fields applied between the electrodes, the alignment of the layers of the composition can become more ordered or more disordered,
the composition has stable states in which the alignment of the layers of the composition are differently ordered including an ordered state, a disordered state and intermediate states, the composition being such that, once switched to a given state by an electric field, it remains substantially in that state when the field is removed,
which composition comprises, in weight %:
                (a) 25-75% in total of at least one siloxane of the general formula I:        
                wherein                    p=1 to 10, e.g. 1 to 3,            q=1 to 12, e.g. 6 to 10,            t=0 or 1,            k=2 or 3,            A is a phenyl or cyclohexyl ring which may be the same or different and are bonded together in para positions,            R=a C1-3 alkyl group, e.g. methyl, which may be the same or different,            X=a C1-12 alkyl group, and            Z═F, Cl, Br, I, CN, NH2, NO2, NMe2, NCS, CH3, or OCH3, CF3, OCF3, CH2F, CHF2 especially CN;                        (b) 0.001-1% in total of at least one quaternary ammonium salt of the general formula II:        
                                    wherein:            T=a methyl group or a silyl or siloxane group and            v=1 to 30, for example v=9 to 19, e.g. myristyl (v=13, T=methyl) or cetyl (v=15 and T=methyl),            R1, R2 and R3, which may be the same or different, are C1-4 alkyl, e.g. methyl or ethyl,            Q− is an oxidatively stable ion, especially a ClO4− ion,                        (c) 20-65% in total of at least one polarisable linear molecule having an alkyl chain, the molecule having the general formula III:D-A′k-Y  (III)         wherein:                    D stands for a C1-16 straight chained alkyl or alkoxy group optionally containing one or more double bonds;            k=2 or 3,            A′ is a phenyl, cyclohexyl, pyrimidine, 1,3-dioxane, or 1,4-bicyclo[2,2,2]octyl ring, wherein each A may be the same or different and are bonded together in para positions, the terminal ring attached to Y optionally being a phenyl and            Y is located in the para position of the terminal ring of the group A′k and is selected from Z (as defined above in connection with Formula I), C1-16 straight chained alkyl, C1-16 straight chained alkoxy, OCHF2, NMe2, CH3, OCOCH3, and COCH3; and                        (d) 2-20%, optionally 5-15, in total of at least one side chain liquid crystal polysiloxane of the general formula IV:        
                                    wherein:            a, b and c each independently have a value of 0 to 100 and are such that a+b+c has an average value in the range 3 to 200, e.g. 5 to 20; and a is such that the chain units of the formula Y—R2SiO—[SiR2—O]a represents 0 to 25 mole percentage of the compound of the general formula IV, and c is such that the units of the formula chain —[SiHR—O]c—R2SiO—Y represents 0 to 15 mole percentage of the compound of the general formula IV,            m=3 to 20, e.g. 4 to 12;            t=0 or 1,            k=2 or 3            A is a phenyl or cyclohexyl ring which may be the same or different and the rings are bonded together in para positions,            R=a C1-3 alkyl group, e.g. methyl, each of which may be the same or different, and            Y=a C1-12 alkyl group, a chromophore or a calamitic liquid crystal group and each of which may be the same or different, and            Z is as defined above in connection with Formula I.                        and wherein the amounts and nature of the components are selected such that the composition possesses smectic A layering and siloxane-rich sub-layering, as detected by X-ray diffraction.        
The compositions of WO/2011/115976 can be used as a basis of the compositions of the present invention and the contents of this specification are incorporated by reference.